1. Field of the Invention
The invention relates to an apparatus, more particularly a freely reciprocating piston engine, comprising a piston which can reciprocate and rotate with in a cylinder and which is dynamically supported and centered in the cylinder by means of a grooved dynamic bearing, the piston having two end faces which are acted upon during operation by different fluid pressures.
2. Description of the Prior Art
In a known engine of the kind mentioned in the opening paragraph (described in an article entitled "Test Results of High Efficiency Stirling Machine Components" by R. J. Vincent, W. D. Rifkin and G. M. Benson, pages 1867-1874, presented on the 17th IECEC in 1982 in Los Angles) which is constructed as a freely reciprocating Stirling engine, the possibility is mentioned of supporting the piston hydrodynamically in the cylinder by causing the piston to rotate as well as reciprocate. It should be noted that at higher speeds of rotation, smooth hydrodynamic bearings become unstable due to the so-called "half frequency whirl". According to this article, this is counteracted by providing the piston with a pattern of spiral grooves.
A disadvantge of the known engine is that the pressure difference applied across the piston leads to a decrease in pressure across the hydrodynamic bearing which introduces a radial dynamic instability of the bearing which is of a nature different from that of the said "half frequency whirl" and occurs especially at low speeds of rotation. As a result, the piston has a tendency to occupy an eccentric position in the cylinder, so that the leakage between the piston and the cylinder becomes considerably larger compared with the situation in which the piston is centrically disposed in the cylinder. The eccentricity also results in increased friction and wear.
A possibility of avoiding this kind of instability is to cause the piston to rotate at a high speed, but for various reasons this is often very unattractive.